FIG. 4 is a circuit diagram showing an example of a conventional switching power supply for a charger. In FIG. 4, reference numeral 130 denotes a semiconductor device for controlling a switching power supply (hereinafter, simply referred to as a semiconductor device). The semiconductor device 130 is constituted by a switching element 101 and a control circuit thereof.
The semiconductor device 130 has, as external input terminals, five terminals of the input terminal (DRAIN) of the switching element 101, an auxiliary power supply voltage input terminal (VCC), an internal circuit power supply terminal (VDD), a feedback signal input terminal (FB), and a GND terminal (GND) of the control circuit that serves as an output terminal of the switching element 101.
Reference numeral 102 denotes a regulator for providing the internal circuit power supply of the semiconductor device 130. The regulator 102 comprises a switch 102A for applying starting current to the VCC and a switch 102C for applying current from the VCC to the VDD.
Reference number 103 denotes a starting constant-current source for feeding starting circuit current. The constant current source feeds starting current to the VCC via the switch 102A upon startup.
Reference numeral 107 denotes a start/stop circuit for controlling start/stop of the semiconductor device 130. The start/stop circuit detects the voltage of the VCC and outputs a signal for stopping the switching operation of the switching element 101 to a NAND circuit 105 when the VCC has a voltage equal to or lower than a given voltage.
Reference numeral 106 denotes a drain current detection circuit for detecting a current applied to the switching element 101. The detection circuit 106 converts a detected current into a voltage signal and outputs the signal to a comparator 108.
Reference numeral 111 denotes a feedback signal control circuit which converts a current signal, which is inputted to the FB terminal, into a voltage signal and outputs the signal to the comparator 108.
The comparator 108 outputs a signal to the reset terminal of an RS flip-flop circuit 110 when an output signal from the feedback signal control 111 and an output signal from the drain current detection circuit 106 are equal to each other.
A clamping circuit 112 is a circuit for determining the maximum value of an output signal from the feedback signal control circuit 111. The clamping circuit 112 determines the maximum value of current applied to the switching element 101 and performs the function of overcurrent protection for the switching element 101.
Reference numeral 109 denotes an oscillation circuit which outputs a maximum duty cycle signal 109A for determining the maximum duty cycle of the switching element 101 and a clock signal 109B for determining an oscillation frequency of the switching element 101. The maximum duty cycle signal 109A is inputted to the NAND circuit 105 and the clock signal 109B is inputted to the set terminal of the RS flip-flop circuit 110.
The output signal of the start/stop circuit 107, the maximum duty cycle signal 109A, and the output signal of the RS flip-flop circuit 110 are inputted to the NAND circuit 105. The output signal of the NAND circuit 105 is inputted to a gate drive circuit 104 to control the switching operation of the switching element 101.
Reference numeral 140 denotes a transformer which has a primary winding 140A, a secondary winding 140C, a secondary auxiliary winding 140B, and a primary auxiliary winding 140D.
A rectifying/smoothing circuit constituted by a diode 131 and a capacitor 132 is connected to the primary auxiliary winding 140D, and is utilized as the auxiliary power supply of the semiconductor device 130. Input is made to the VCC.
Reference numeral 133 denotes a capacitor for stabilizing the VDD. Reference numeral 135 denotes a control signal transmission circuit for transmitting a control signal from the secondary side to the primary side. The transmission circuit 135 is constituted by a phototransistor 135A and a light-emitting diode 135B. The collector of the phototransistor 135A is connected to the VDD and the emitter of the phototransistor 135A is connected to the FB.
A rectifying/smoothing circuit constituted by a diode 152 and a capacitor 153 is connected to the secondary winding 140C. The rectifying/smoothing circuit is further connected to a load 157. A rectifying/smoothing circuit constituted by a diode 150 and a capacitor 151 is connected to the secondary auxiliary winding 140B and feeds current to the light-emitting diode 135B and a secondary control circuit 158.
The secondary control circuit 158 is constituted by a constant voltage control circuit 159 and a constant current control circuit 160. The constant voltage control circuit 159 is fed with voltage divided by detection resistors 154 and 155 of secondary output voltage Vo and controls current applied to the light-emitting diode 135B so as to have a constant secondary output voltage Vo. The constant current control circuit 160 operates when current applied to an output current detection resistor 156 becomes equal to or higher than a give current, and the constant current control circuit 160 controls current applied to the light-emitting diode 135B so as to have a constant output current Io.
Referring to FIGS. 4 and 5, the operations of the switching power supply configured thus will be described below. FIG. 5 is a time chart for explaining the operation waveforms of the above-described parts.
In FIG. 4, a direct-current voltage VIN generated by performing rectification and smoothing on, for example, a commercial alternating-current power supply is inputted to the input terminals. The VIN is applied to the DRAIN terminal of the semiconductor device 130 via the primary winding 140A of the transformer 140. Then, starting current generated by the starting constant-current source 103 is applied to charge the capacitor 132, which is connected to the VCC, via the switch 102A in the regulator 102, so that the voltage of the VCC is increased. The switch 102C in the regulator 102 operates such that the VDD has a constant voltage. Thus, some of the starting current charges the capacitor 133, which is connected to the VDD, via the switch 102C, so that the voltage of the VDD is also increased.
When the voltage of the VCC increases and reaches the starting voltage set by the start/stop circuit 107, the switching operation of the switching element 101 is started. When the switching operation is started, energy is supplied to the windings of the transformer 140 and thus current is applied to the secondary winding 140C, the secondary auxiliary winding 140B, and the primary auxiliary winding 140D.
Current applied to the secondary winding 140C is rectified and smoothed by the diode 152 and the capacitor 153 into direct-current power, which supplies power to the load 157. The output voltage Vo is gradually increased by repeating the switching operation. When the output voltage Vo reaches a voltage set by the output voltage detection resistors 154 and 155, current applied to the light-emitting diode 135B is increased in response to a signal from the constant voltage control circuit 159. Then, current applied to the phototransistor 135A is increased and current applied to the FB terminal is also increased. When the current of the FB terminal is increased, voltage inputted to the comparator 108 decreases, thereby reducing drain current applied to the switching element 101. With such a negative feedback, the output voltage Vo is stabilized.
Current applied to the primary auxiliary winding 140D is rectified and smoothed by the diode 131 and the capacitor 132, is utilized as the auxiliary power supply of the semiconductor device 130, and feeds current to the VCC terminal. Once the VCC reaches the starting voltage, the switch 102A in the regulator 102 is turned off. Thus, the current of the semiconductor device 130 is fed from the primary auxiliary winding 140D after startup. The polarity of the primary auxiliary winding 140D is the same as the secondary winding 140C and thus the VCC has a voltage proportionate to the output voltage Vo.
Current applied to the secondary auxiliary winding 140B is rectified and smoothed by the diode 150 and the capacitor 151 and is utilized as the power supply of the secondary control circuit 158 and the light-emitting diode 135B. The polarity of the secondary auxiliary winding 140B is the same as the primary winding 140A and thus the secondary auxiliary winding has a voltage proportionate to the input voltage VIN.
After the output voltage Vo is stabilized, the output current Io applied to the load 157 is increased. When current applied to the output current detection resistor 156 reaches a given value, the constant current control circuit 160 is operated to increase current applied to the light-emitting diode 135B. Then, current applied to the phototransistor 135A is increased and current applied to the FB terminal is also increased. When the current of the FB terminal is increased, voltage inputted to the comparator 108 decreases, thereby reducing drain current applied to the switching element 101. With such a negative feedback, control is performed so as to have a constant output current. Thus, in the case of a negative current equal to or higher than a given current, a constant-current drooping characteristic is obtained with a constant output current and a reduced output voltage.
When a load is further applied, the output voltage Vo is further reduced. At this point of time, a primary auxiliary winding voltage VCC is also reduced. Then, at a voltage equal to or lower than a stop voltage set by the start/stop circuit 107, the switching operation of the switching element 101 is stopped. Then, the switch 102A in the regulator 102 is brought into conduction again. Thus, starting current is applied by the starting constant-current source 103 and the VCC increases again. When the VCC reaches a starting voltage set by the start/stop circuit 107, the switching operation of the switching element 101 is resumed. Then, the switch 102A in the regulator 102 is turned off. When the VCC decreases and reaches the stop voltage, the switching operation is stopped. Namely, in an overload state having a short-circuit load and so on, an intermittent oscillation occurs which repeats the switching operation and the stopping operation. Therefore, the output current voltage characteristic of FIG. 4 is illustrated as FIG. 9 where an intermittent oscillation occurs when an output voltage droops to a given voltage or below.
FIG. 6 shows a variation of FIG. 4. FIG. 6 is different from FIG. 4 only in the polarity of a primary auxiliary winding 140E. A primary auxiliary winding voltage VCC is proportionate to an input voltage VIN.
Referring to FIG. 7, the operations of a switching power supply configured as FIG. 6 will be described below. FIG. 7 is a time chart for explaining the operation waveforms of the above-described parts of FIG. 6.
The operations of FIG. 6 are different from those of FIG. 4 only in the event of an overload and thus the explanation of normal operations is omitted.
In the event of an overload, an output voltage Vo is reduced, whereas the primary auxiliary winding voltage VCC is not reduced. Thus, the switching operation of a semiconductor device 130 is continued. For this reason, even in the event of a short-circuit load, a current determined by a secondary current limiting resistor 156 is applied. Therefore, the output current voltage characteristic of FIG. 6 is illustrated as FIG. 10 where an output voltage droops while a constant current is maintained.
In general a switching power supply requires a protecting function for a short-circuit load. It is desired that short-circuit load current be minimized so as to prevent a switching power supply component from generating heat or being damaged even when the short circuit of the load is continued. Hence, the primary side normally has an overcurrent protecting function for stopping a switching operation when current applied to a switching element becomes equal to or higher than a given current.
However, a switching power supply for a charger has to be constituted by a secondary measurement current control circuit for charging a battery with a constant current. Further, when the secondary measurement current control circuit is operated, that is, when a constant current droops, the overcurrent protecting function on the primary side is not performed.
Therefore, the switching power supply for a charger cannot effectively perform the function of overcurrent protection on the primary side in the event of a short-circuit load. The conventional switching power supply for a charger that is shown in FIG. 4 causes an intermittent oscillation in the event of a short-circuit load but has a large load current applied during the oscillating period of the intermittent oscillation, resulting in an insufficient protecting function for a short-circuit load.
Moreover, in the conventional switching power supply of FIG. 6, a current with a short-circuit load is equal to a drooping current value. Thus, it is not possible to reduce a short-circuit load current.
Hence, in order to reduce the short-circuit load current of the switching power supply for a charger, it is necessary to provide another short-circuit load protection circuit on the secondary side, increasing the cost and the number of components.